Depending on the state of discharge, a Noco type of battery booster may work, but I suspect from your comments the starter battery is goosed. A multi meter reading of all battery's standing volts is a good place to start.So are we saying the S3 light is relating to the electrical system then? I will get the vehicle started and then go from there, yes I have a multimeter to test the main vehicle battery and the leisure one, but with it being parked 15 miles away from home I can't leave a battery charger on as no facility available on the site.
there is a solar panel on the roof of the vehicle but not sure if it links into the panel above the door as it seems to have its own independent panel with a couple of usb ports in it.So a multiple of issues. Start [ no pun intended] with the starter battery. Have you a multimeter to see what the state of charge of the starter battery is now? Have you a good charger to recharge the starter battery? How old is the starter battery? It may well be time for a new one.
Has something been left switched on while the vans been stood up, 12v supply? Is there any solar input, or hook up to it ?
That was the first big mistake., I loaned the vehicle to a friend
No " battery master" fitted ? If not then the starter battery has been drained, probably to a level it may not recover from.there is a solar panel on the roof of the vehicle but not sure if it links into the panel above the door as it seems to have its own independent panel with a couple of usb ports in it.
sorry not sure what a battery master is? I've got a 2.0tdi Passat and the motorhome is a 2.3 diesel Fiat Ducato Rollerteam 707, going to try and get it started and then I can possibly check the water levels in the tanks but would be good to find out what the S3 on the control panel relates too.No " battery master" fitted ? If not then the starter battery has been drained, probably to a level it may not recover from.
How difficult is access to the starter battery for removal, as it sounds like you may well have hoik that out?
Getting the van started is probably the best starting place, once mobile other options open up.
What size battery is in your other vehicle? You maybe able to jump assist the vans battery, but that to can lead to issues if not done correctly!!